The present invention relates to a method for determining the amount of charge which can be drawn from a storage battery. The present invention also relates to a monitoring device for a storage battery having measurement means for measurement of the battery voltage and/or battery currents, and also having evaluation means.
The use of rechargeable storage batteries, in particular when they are being discharged and charged, results in wear. In addition, other operating conditions also exist which speed up the wear to electrochemical energy stores. In the case of a lead-acid rechargeable battery, for example, these include the overall operating life (i.e., the entire time which has passed since the battery was first brought into use, including the periods in which the battery had no electrical load applied to it).
This wear is further exacerbated by increased temperatures. Increased temperatures may, however, not only speed up the wear during periods in which no electrical load is applied, but also may exacerbate the wear caused by cyclic discharging and charging operations.
In the case of a storage battery, the wear is expressed, inter alia, by a reduction in the storage capacity for electrical charge, so that the amount of charge that can be drawn starting from the full state of charge cannot easily be determined.
The amount of charge QR which can be drawn from the storage battery is in this case the amount of charge which can still be drawn in nominal conditions, starting from the present state of the storage battery. When the storage battery is in the new state, the sum of the amount of charge which can be drawn and the amount of charge which has been discharged is the storage capacity in the new state.
The storage capacity in the new state is the actual storage capacity of a new, unused energy store.
The amount of charge discharged is the amount of charge which must be drawn in nominal conditions from a storage battery starting from the fully charged state, in order to reach the present state of charge.
The present storage capacity is the amount of charge expressed in amp-hours (Ah) which can be drawn in nominal conditions from a correctly fully charged energy store. This variable changes with the time during which the storage battery has been in use, generally with a falling tendency.
The nominal storage capacity is defined as the nominal value of the storage capacity as stated by the manufacturer of the storage battery. The reduction in the storage capacity is regarded as the difference between the present storage capacity and the storage capacity in the new state.
In this context, U.S. Pat. No. 5,761,072 describes a method for determining the capacity of a storage battery, in which a fast current is determined by means of a filter and a slow current is determined by averaging by means of integration. The values for the fast current and slow current are used in what is referred to as a Peukert relationship in order to determine the capacity for a fast current and for a slow current. These capacities are weighted, and an overall capacity is calculated from them.
DE 93 21 638 describes an electronic battery tester for testing an electrochemical cell or battery, which has a dynamic parameter, such as a conductance value or a resistance. The battery tester has a device for measurement of this dynamic parameter, for measurement of the no-load voltage and for correction of the measured dynamic parameter value with respect to the state of charge, by matching the no-load voltage.
U.S. Pat. No. 5,680,050 discloses a method for battery state identification, in which a correction value is determined on the basis of an average discharge, for a time period which is longer than the time period which changes the discharge polarization to a steady state. The capacity of the battery in the fully charged state is multiplied by this correction value, and the energy consumption of the battery is subtracted from the available discharge capacity in order to determine the available spare capacity.
The above-mentioned methods and apparatuses determine present electrical characteristic values for a storage battery in terms of a conductance value or internal resistance or determine the discharge capacity which can be drawn from a storage battery as a function of discharge current, although this has the disadvantage that the capacity of the fully charged storage battery must be known.
DE 691 31 276 T2 discloses an electronic tester for assessment of the percentage energy capacity of a storage battery or a battery cell. In this method, the dynamic conductance value is determined and is related to a reference conductance value which corresponds to the dynamic conductance value of a battery or battery cell with 100% capacity. However, this tester cannot be used to determine the total capacity of a fully charged storage battery or to distinguish whether any change has taken place in the capacity due to aging or due to normal discharging. It is not yet possible to use the time response of the measurement values over a lengthy period of time to obtain information relating to the state of wear of the storage battery. Furthermore, the dynamic conductance value is only of limited use as a measure for battery wear.
EP 0 516 336 B1 discloses a method for determining the amount of charge which can be drawn and the remaining capacity of a storage battery, in which the imaginary part of a response signal profile in the impedance spectra domain is regarded as a function of the root of a sinusoidal stimulus frequency. This is extrapolated to the point at which the response signal profile in the impedance spectra domain intersects the imaginary axis. The remaining capacity is deduced from the extrapolated value that is obtained from this. In this case, a search is carried out for the radio-frequency intersection of a Warburg straight line, and this is used as a measure for the amount of charge which can be drawn.
There is thus a need to estimate the amount of charge which can be drawn from a storage battery during operation. There is also a need to provide an improved method for determining the amount of charge which can still be drawn from a storage battery when the storage battery is no longer in the new state. There is also a need for a monitoring device which has measurement means and evaluation means (e.g., in the form of a program which can be run on a processor) designed for carrying out the method.